In a supercharger incorporated in an internal combustion engine of, for example, an automobile, ships and vessels, a turbine impeller at an exhaust side is caused to rotate with utilization of exhaust gas from an internal combustion engine, thereby rotating a coaxial compressor impeller at an intake side, or by rotating the coaxial compressor impeller, to suck and compress an outside air and to supply the compressed air to the internal combustion engine to increase an output of the internal combustion engine.
Since a turbine impeller used for the supercharger described above is exposed to high temperature exhaust gas discharged from an internal combustion engine, super alloys of Ni-base, Co-base, Fe-base, etc. proposed in, for example, JP-A-58-70961 (Patent Publication 1) have been conventionally used therefor. In recent years, titanium alloys and aluminum alloys have been also used. On the other hand, a compressor impeller is positioned in a location at which an outside air is sucked, and used in a temperature environment in the order of 100° C. to 150° C. Therefore, aluminum alloys conventionally have been used much for the compressor impeller instead of alloys having heat high resistance like as super alloys being used for the turbine impeller described above.
In recent years, various examinations have been made for further high speed rotation of a turbine impeller and a compressor impeller with a view to an improvement in combustion efficiency of an internal combustion engine. In rotating an impeller at high speed, it is desired that, in particular, a compressor impeller be high in strength (referred below to as specific strength) per unit density, that is, lightweight and high in strength. Also, it is predicted that a temperature environment at the time of high speed rotation will rise to a temperature beyond 180° C. to 200° C., and it is therefore desired that the impeller have a favorable toughness, be further high in strength, and can be maintained high in strength even when a temperature environment exceeds 200° C.
In the light of such background, a compressor impeller proposed by, for example, JP-A-20003-94148 (Patent Publication 2) is being put to practical use, which is made of a titanium alloy to be able to be made more lightweight than that made of the Ni heat resistant alloy, etc. and to be higher in strength than that made of a conventional aluminum alloy.
Generally, a compressor impeller is complex in shape such that a plurality of blade parts having an aerodynamically curved surface are arranged radially around a hub shaft part on a hub surface of a hub disk part extending radially of the hub shaft part being a rotational center axle. Also, there are also existent an impeller including a blade part composed of full blades and splitter blades and an impeller having a complex shape, in which an undercut extends radially outwardly of a hub shaft part.
A compressor impeller having such complex shape is formed by measures such as machining, by which a blade part is cut from an impeller material, deformation and straightening of a blade part after an impeller material having a shape affording casting is once formed, as proposed by JP-A-57-171004 (Patent Publication 3), or the like. Also, there is also existent a method, in which an sacrificial pattern having a blade part and a hub part of an impeller made integral is formed in a die by means of the plaster mold process, the lost wax casting process and used to fabricate a casting mold, and a molten metal is cast into the casting mold to form an impeller. In this case, for example, the Patent Document 2 and JP-A-2002-113749 (Patent Document 4) propose a die structure to release blade parts from a die, in which an sacrificial pattern is formed.    Patent Publication 1: JP-A-58-70961    Patent Publication 2: JP-A-2003-94148    Patent Publication 3: JP-A-57-171004    Patent Publication 4: JP-A-2002-113749